An image display device is roughly classified into an impulse type display device and a hold type display device. In a CRT (Cathode Ray Tube) mentioned of as an example of an impulse type display device, a screen is scanned by an electron gun and a display is produced only in pixels that electron beams have reached. In contrast to this, in a liquid crystal display device or an organic electroluminescence display device mentioned of as a hold type display device, a frame of an image signal is updated at a fixed period and when a display of an image of a certain first frame is specified, the display of the image of the first frame is held until a display of an image of the next second frame is specified. Compared to an impulse type display device, a hold type display device has various characteristics, such as that image distortion is unlikely to occur.
However, a liquid crystal display device has a problem that response is slow. That is, it takes time for an actual display value in a liquid crystal display device to reach a target display value after the target display value of an image of a certain frame is specified. There may be a case where the required time exceeds a period at which a frame is updated. Consequently, when a motion picture in which images changes rapidly is displayed on the screen of a liquid crystal display device, there may be a case where blur appears in the motion picture.
As a technique intended to solve such a problem, the overdrive technique is publicly known. According to the overdrive technique, when a certain pixel on the screen of a liquid crystal display device is focused on, if image data G2 corresponding to a target display value in the next second frame is different from image data (luminance) G1 corresponding to a target display value in a certain first frame, the image data G2 is corrected and then, corrected image data G2′ is given to the liquid crystal display device. At the time of the correction, when “G1<G2”, G2 is corrected so that “G2<G2′” and when “G1>G2”, then corrected so that “G2>G2′”. By providing an image signal processing device that outputs an image signal to a liquid crystal display device after processing image data of each frame of the image signal as described above, it is made possible for the actual display value to reach the target display value quickly in the liquid crystal display device.
There have been made various proposals relating to the overdrive technique. In the invention disclosed in patent document 1, in order to acquire the corrected image data G2′ by performing predetermined processing based on the image data G1 of a certain first frame and the image data G2 of the next second frame, image data of each frame is encoded and the encoded image data of each frame is delayed by a delay part by a period of time corresponding to one frame. Then, the encoded image data of the frame to be output from the delay part after being decoded by a first decoding part is taken as image data of the first frame and at the same time, the encoded image data of the frame to be input to the delay part after being decoded by a second decoding part is taken as image data of the second frame, and based on the decoded image data of the first frame and that of the second frame, respectively, the corrected image data G2′ is acquired.
With the overdrive technique, image data corresponding to one frame is delayed by the delay part after being encoded, and therefore, the capacity of a memory used as the delay part can be reduced. Further, to the image data of the first frame and the second frame used to acquire the corrected image data G2′, an error substantially the same is attached at the time of encoding processing and decoding processing, and therefore, it is thought that the corrected image data G2′ is not affected by the error.    Patent document 1: Japanese Unexamined Patent Publication (Kokai) No. 2003-202845